Marcus Aurelius Antoninus (born Lucius Septimius Bassianus, 4 April 188 – 8 April 217), better known by his nickname Caracalla, was Roman emperor from 198 to 217 AD. He was a member of the Severan dynasty, the elder son of Emperor Septimius Severus and Empress Julia Domna.

He was renamed Marcus Aurelius Antoninus at the age of seven as part of his father's attempt at union with the families of Antoninus Pius and Marcus Aurelius. According to the 4th century historian Aurelius Victor in his Epitome de Caesaribus, he became known by the agnomen "Caracalla" after a Gallic hooded tunic that he habitually wore and made fashionable

Proclaimed co-ruler by his father in 198, he reigned jointly with his brother Geta, co-emperor from 209, after their father's death in 211. His brother was murdered by the Praetorian Guard later that year under orders from Caracalla, who then reigned afterwards as sole ruler of the Roman Empire. Caracalla found administration to be mundane, leaving those responsibilities to his mother. While Caracalla was mustering and training troops for his planned Persian invasion, Julia remained in Rome, administering the empire. Julia's growing influence in state affairs was the beginning of a trend of emperors' mothers having influence, which continued throughout the Severan dynasty.

Caracalla's reign featured domestic instability and external invasions by the Germanic peoples.

Caracalla's reign became notable for the Antonine Constitution (Latin: Constitutio Antoniniana), also known as the Edict of Caracalla, which granted Roman citizenship to all free men throughout the Roman Empire. One purpose for Caracalla issuing the edict was the desire to increase state revenue; at the time, Rome was in a difficult financial situation and needed to pay for the new pay raises and benefits that were being conferred on the military. The edict widened the obligation for public service and gave increased revenue through the inheritance and emancipation taxes that only had to be paid by Roman citizens. Domestically, Caracalla became known for the construction of the Baths of Caracalla, which became the second-largest baths in Rome; for the introduction of a new Roman currency named the antoninianus, a sort of double denarius; and for the massacres he ordered, both in Rome and elsewhere in the empire. In 216, Caracalla began a campaign against the Parthian Empire. He did not see this campaign through to completion due to his assassination by a disaffected soldier in 217. Macrinus succeeded him as emperor three days later.

The ancient sources portray Caracalla as a tyrant and as a cruel leader, an image that has survived into modernity. His contemporaries Cassius Dio (c. 155 – c. 235) and Herodian (c. 170 – c. 240) present him as a soldier first and an emperor second. In the 12th century, Geoffrey of Monmouth started the legend of Caracalla's role as king of Britain. Later, in the 18th century, the works of French painters revived images of Caracalla due to apparent parallels between Caracalla's tyranny and that ascribed to King Louis XVI (r. 1774–1792). Modern works continue to portray Caracalla as an evil ruler, painting him as one of the most tyrannical of all Roman emperors.